This invention relates generally to turbocharged diesel engines that propel motor vehicles and are equipped with variable valve actuation apparatus, and in particular to a control strategy for concurrently reducing particulate emission and increasing engine torque during transient operation of an engine, such as when the engine is being accelerated to accelerate the vehicle.
A turbocharger is one type of device that is used to supercharge an internal combustion engine. A diesel engine that is supercharged by a turbocharger is sometimes referred to as a turbocharged diesel. A turbocharger comprises a turbine that is powered by engine exhaust gas and coupled by a shaft to operate a compressor that boosts pressure in the engine air intake system downstream of the compressor. Boost is controlled by controlling turbine operation.
A turbocharged diesel engine operating as the prime mover of a motor vehicle can provide certain advantages that are unavailable with other types of vehicle powerplants. For example, increasing boost allows increased fueling of the engine, enabling the engine to develop more torque than without turbocharging. Known devices that enable an engine to deliver increased torque for vehicle acceleration include variable geometry/nozzle turbochargers and superchargers.
A successful strategy for controlling operation of a turbocharged diesel engine in a vehicle should enable the engine to deliver torque appropriate to the manner in which the vehicle is being driven without generating unacceptable levels of tailpipe emissions. Transient operating conditions, like those occurring when a vehicle is accelerating, may have significant influence on tail pipe emission, and so special attention may have to be given to engine operation during those events to assure compliance with applicable emission limits. Attainment of that objective at the expense of compromising engine torque would be undesirable.
Certain commonly assigned patent applications of the inventors have disclosed that certain turbocharged diesel engines, especially engines that have variable valve actuation, can develop increased torque without undesirable consequences on tailpipe emissions, such as smoke in the engine exhaust. That improvement is achieved by certain conjunctive control of: 1) timing at which the engine intake and/or exhaust valves open during an engine operating cycle; and 2) engine fueling.
The present invention concerns a discovery that enables such an engine to develop increased engine torque during transient operation, such as during acceleration, without unacceptable effect on tailpipe emissions, especially particulate emissions.
Because certain principles of the present invention include changing the time in the engine operating cycle when the exhaust valves open, the engine must have an appropriate mechanism for each exhaust valve. An example of such a mechanism comprises an electric actuator for opening and closing an exhaust valve in accordance with an electric signal applied to the actuator. Such an engine is sometimes referred to as a camless engine, particularly where the engine intake valves are also controlled by electric actuators. When the inventive strategy is invoked, the timing of the opening of each exhaust valve during the engine cycle is increasingly retarded.
By retarding exhaust valve opening, the in-cylinder burning time for particulates is increased, and this reduces particulate emission. Retarding the exhaust valve opening has also been discovered to provide increased energy input to the turbocharger compressor, thereby increasing boost. As boost increases and smoke decreases, engine fueling is also increased to develop increased engine torque so that the additional fueling is not adverse to tailpipe emissions in any significant way. In this way, the turbocharger is forced toward operating at its performance limit, thereby enabling the engine to develop a corresponding torque that is greater than the torque that would otherwise be achieved.
If a consequence of retarding exhaust valve opening in conjunction with increasing engine fueling creates incipient turbocharger surging, bleeding charge air according to the inventors"" prior disclosure, may be employed to counteract, or prevent, the incipient surging. Because a turbocharger is typically designed with high speed, rather than low speed, operation in mind, surging may be a concern only during an early portion of the acceleration time when turbocharger speed is still relatively low.
A primary aspect of the present invention relates to a novel strategy for controlling exhaust valve opening in a turbo-diesel engine that has a variable valve actuation apparatus. When the engine is being accelerated, the engine control system causes the exhaust valves to open at a later time during the engine cycle than they would in an engine that has a camshaft operating the exhaust valves. The extent to which the control system retards exhaust valve opening is a function of one or more variables that, for example, are used as inputs to a look-up table to yield values for controlling exhaust valve opening.
The disclosed strategy is implemented via a processor-based engine control. The data is processed according to a software algorithm that is executed by the processor to develop data for a control signal that is applied to the exhaust valve actuators to control exhaust valve timing. Engine fueling is also increased, consistent with increasing turbocharger boost to enable the engine to develop increasing torque consistent with limiting particulate tailpipe emissions.
One general aspect of the claimed invention relates to an internal combustion engine comprising an intake system through which charge air is delivered to an intake manifold of the engine, including a turbocharger that comprises a compressor operated by exhaust gases from the engine for creating compressed charge air that provides boost in the intake manifold. A control controls the opening of engine exhaust valves in relation to an engine operating cycle and fueling of the engine in relation to the engine operating cycle. During transient operation of the engine, the control increasingly retards exhaust valve opening in relation to the engine operating cycle to cause the turbocharger to increase intake manifold pressure and increases engine fueling in relation to the increased intake manifold pressure.
Another general aspect of the claimed invention relates a method for an engine as just described wherein in response to initiation of transient operation of the engine, exhaust valve opening is increasingly retarded in relation to the engine operating cycle to cause the turbocharger to increase intake manifold pressure and engine fueling is increased in relation to the increased intake manifold pressure.